1. Field of Invention
The invention relates to a ferrofluid having low optical density. Further, the invention relates to a method of preparing the ferrofluid. More particularly, the invention relates to a method of preparing an aqueous ferrofluid.
More specifically, the invention relates to a method for the preparation of colored ferrofluids using various colorants, dyes or pigments. Finally, the invention relates to xerographic magnetic liquid toners, colored xerographic magnetic liquid toners and liquid ink compositions and methods of preparation thereof.
2. Discussion of the Prior Art
The preparation of magnetic fluids is, in general, a very time intensive process most simply done by grinding a magnetic material such as magnetite, Fe.sub.3 O.sub.4, in a suitable liquid vehicle in the presence of a dispersing agent or surfactant to obtain a stable colloidal magnetic fluid. This general preparation is described in detail in Rosensweig & Kaiser "Study of Paramagnetic Liquids," NASA Document N68-14205, Wilmington, Mass. 1967 and IEEE Transactions on Magnetics, Vol. MAG-16, No. 2, March 1980.
In a typical grinding or milling operation to produce magnetic fluids, grinding or milling times of 120-2900 hours (five days to four months) are required. The problem is in producing small enough magnetic particles to enable the formation of a stable colloid. The use of dispersing agents or surfactants is also a problem in that the correct or enabling surfactant must be found empirically. Furthermore, the surfactant may degrade or cause adverse chemical reactions in the magnetic fluid during its application.
In addition, prior art magnetic fluids are typically, by their very nature, black or very dark brown in color and therefore highly absorbent in the visible region of the spectrum. At the heart of such materials are magnetic materials such as iron, cobalt or nickel particles, iron oxide such as Fe.sub.3 O.sub.4, and the like, generally in an assigned range of about 10-1000 .ANG.. These prior art magnetic fluids are not particularly useful in applications requiring low optical density as they are highly absorbing. Examples of such applications include those requiring high magnetism and low optical density or high optical transmission, particularly in the visible and near infrared region of the spectrum, such as, magneto-optic and electro-optic effects.
Moreover, if a fluid with these magnetic properties is required to be colored, i.e. by mixing it with various colorants, dyes or pigments, the brown, black or muddy appearance of the prior art magnetic fluids produced a colored magnetic fluid which was also brown, black or muddy in appearance. Thus, applications requiring brightly colored fluids that are magnetic, for example, inks and toners were not possible using the prior art magnetic fluids. Moreover, when colorant was added to prior art magnetic fluids, a mixture of dye and magnetic fluid was formed. If a single component colored fluid was required, its formation was not possible using prior art magnetic fluids.